Perhaps tar generation has been switched to using git-archive instead ofgit-tar-tree? There's an, admittedly, subtle difference in how the twohandle prefixes/basedirs. The following two commands do the same:

$ git-tar-tree rev basedir $ git-archive --prefix=basedir/ rev

If you use --prefix and you want a base directory then you have toprovide your own slash (basedir = prefix + path_separator).